function main(){
    //1.js上获取canvas
    //2.此处说法是获取canvas绘制的上下文
    //3.用这个上下文调用函数来绘制

    //1.
    const canvas = document.getElementById('mCanvas')
    if(!canvas){
        console.log('could not run THE WEBGL in the Web Brower')
    }

    //2.
    let context = canvas.getContext('2d')
    if(!context){
        console.log('let context error')
    }
    console.log('let see what is 2d context',context)
    
    //3.
    context.fillStyle = 'rgba(0,0,255,0.5)'
    context.fillRect(120,10,150,150)
}